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Abstract

IEEE 802.11 DCF (Distributed Coordination Function) is a popular protocol used for
the physical and MAC layers in most ad hoc networks. DCF employs carrier sense multiple
access with collision avoidance (CSMA/CA) and a binary slotted exponential backo . It has
been observed that the hidden and exposed terminal problems among stations are responsible
for DCF's performance-degradation. The issue of fairness is also a major contributor to
its low-grade performance. Hence, the e ectiveness of 802.11 DCF mechanism in ad hoc
networks has attracted many research studies.
There has been many proposals of Collision Avoidance schemes to compete against
the IEEE 802.11 DCF. This is because the performance of the MAC layer directly impacts
the performance of higher-layer protocols and hence the entire network. An evaluation of
these schemes will be helpful in understanding the limitations of wireless ad hoc networks.
In this thesis, we survey various collision avoidance schemes, classify them based on their
mechanism, and then provide a comparative study of the selected schemes based on their
performance. They are evaluated in a Chain topology, a Pair topology and a Random
topology with static environment to provide extensive results on their Throughput, Fairness,
Collision and Delay performance. Based on the evaluation, we conclude that GDCF (Gentle
DCF) is the best scheme that has lesser collisions with improved throughput and fairness.
A comparison with the legacy CSMA/CA suggests that these proposed schemes do tend to
be promising and would inspire future researchers who are interested to nd solutions to the
age-old collision and fairness issues in ad-hoc networks.